Overcurrent protection circuit

ABSTRACT

An overcurrent protection circuit limits a monitoring target current to or below a limit current value IOCP. For example, it can operate so as to give the limit current value IOCP, when temperature Tj is lower than a threshold value Tx, a flat temperature response and, when temperature Tj is higher than the threshold value Tx, a negative temperature response. For another example, it can operate so as to set the limit current value IOCP, when temperature Tj is lower than a threshold value Tx, at a first limit current value IOCP having a flat temperature response and, when temperature Tj is higher than the threshold value Tx, at a second limit current value IOCP having a flat temperature response and lower than the first limit current value IOCP.

TECHNICAL FIELD

The invention disclosed herein relates to an overcurrent protectioncircuit.

BACKGROUND ART

Many proposals have conventionally been made as to overcurrentprotection circuits for limiting a monitoring target current to or belowa limit current value.

One example of known technology related to what has just been mentionedis seen in Patent Document 1 identified below.

CITATION LIST Patent Literature

Patent Document 1: Japanese unexamined patent application publicationNo. 2005-328606

SUMMARY OF INVENTION Technical Problem

Inconveniently, conventional overcurrent protection circuits leave roomfor further studies in terms of optimizing the limit current value inaccordance with temperature.

In view of the above-mentioned problems encountered by the preventinventors, an object of the invention disclosed herein is to provide anovercurrent protection circuit that offers adequate overcurrentprotection from a low to high temperature range.

Solution to Problem

According to one aspect of what is disclosed herein, an overcurrentprotection circuit for limiting a monitoring target current to or belowa limit current value is configured to give the limit current value,when temperature is lower than a threshold value, a flat temperatureresponse and, when temperature is higher than the threshold value, anegative temperature response. (A first configuration.)

The overcurrent protection circuit of the first configuration describedabove may include: a current detector configured to detect themonitoring target current to generate a detection signal with a flattemperature response; and an amplifier or comparator configured to havea first input terminal fed with the detection signal, a second inputterminal fed with a first reference signal with a flat temperatureresponse, and a third input terminal fed with a second reference signalwith a negative temperature response. The amplifier or comparator may beconfigured to generate an overcurrent protection signal in accordancewith the difference between, or the result of comparison between, one ofthe first and second reference signals and the detection signal. (Asecond configuration.)

The overcurrent protection circuit of the first configuration describedabove may include: a current detector configured to detect themonitoring target current to generate a detection signal with a flattemperature response; a first amplifier or first comparator configuredto generate a first overcurrent protection signal in accordance with thedifference between, or the result of comparison between, a firstreference signal with a flat temperature response and the detectionsignal; and a second amplifier or second comparator configured togenerate a second overcurrent protection signal in accordance with thedifference between, or the result of comparison between, a secondreference signal with a negative temperature response and the detectionsignal. (A third configuration.)

In the overcurrent protection circuit of the third configurationdescribed above, the first amplifier or first comparator may have lowercurrent consumption than the second amplifier or second comparator andthe second amplifier or second comparator may have faster response thanthe first amplifier or first comparator. (A fourth configuration.)

The overcurrent protection circuit of any of the second to fourthconfigurations described above may further include: a first referencesignal generator configured to generate the first reference signal byusing a band-gap voltage. (A fifth configuration.)

The overcurrent protection circuit of any of the second to fifthconfigurations described above may further include: a second referencesignal generator configured to generate the second reference signal byusing the forward drop voltage across a diode. (A sixth configuration.)

In the overcurrent protection circuit of any of the second to sixthconfigurations described above, the current detector may be configuredto generate the detection signal by passing a mirror currentproportional to the monitoring target current through a sense resistorand thereby subjecting the mirror current to current-voltage conversion.(A seventh configuration.)

The overcurrent protection circuit of the first configuration describedabove may include: a first current detector configured to detect themonitoring target current to generate a first detection signal with aflat temperature response; a second current detector configured todetect the monitoring target current to generate a second detectionsignal with a positive temperature response; and an amplifier orcomparator configured to have a first input terminal fed with the firstdetection signal, a second input terminal fed with the second detectionsignal, and a third input terminal fed with a reference signal with aflat temperature response. The amplifier or comparator may be configuredto generate an overcurrent protection signal in accordance with thedifference between, or the result of comparison between, one of thefirst and second detection signals and the reference signal. (An eighthconfiguration.)

The overcurrent protection circuit of the first configuration describedabove may include: a first current detector configured to detect themonitoring target current to generate a first detection signal with aflat temperature response; a second current detector configured todetect the monitoring target current to generate a second detectionsignal with a positive temperature response; a first amplifier or firstcomparator configured to generate a first overcurrent protection signalin accordance with the difference between, or the result of comparisonbetween, a reference signal with a flat temperature response and thefirst detection signal; and a second amplifier or second comparatorconfigured to generate a second overcurrent protection signal inaccordance with the difference between, or the result of comparisonbetween, the reference signal and the second detection signal. (A ninthconfiguration.)

According to another aspect of what is disclosed herein, an overcurrentprotection circuit for limiting a monitoring target current to or belowa limit current value is configured to set the limit current value, whentemperature is lower than a threshold value, at a first limit currentvalue that has a flat temperature response and, when temperature ishigher than a threshold value, at a second limit current value that hasa flat temperature response and that is lower than the first limitcurrent value. (A tenth configuration.)

Advantageous Effects of Invention

According to the invention disclosed herein, it is possible to providean overcurrent protection circuit that offers adequate overcurrentprotection from a low to high temperature range.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a linear power supply as a comparativeexample.

FIG. 2 is a diagram showing common overcurrent protection operation.

FIG. 3 is a diagram showing the temperature response (a first pattern)of the limit current value in the comparative example.

FIG. 4 is a diagram showing the temperature response (a second pattern)of the limit current value in the comparative example.

FIG. 5 is a diagram showing a plot of common heat sinking.

FIG. 6 is a diagram showing a linear power supply according to a firstembodiment.

FIG. 7 is a diagram showing the temperature response of the limitcurrent value in the first embodiment.

FIG. 8 is a diagram showing a linear power supply according to a secondembodiment.

FIG. 9 is a diagram showing the temperature responses of referencesignals and the limit current value in the second embodiment.

FIG. 10 is a diagram showing a linear power supply according to a thirdembodiment.

FIG. 11 is a diagram showing a linear power supply according to a fourthembodiment.

FIG. 12 is a diagram showing the temperature responses of detectionsignals, a reference signal, and the limit current value in the fourthembodiment.

FIG. 13 is a diagram showing a linear power supply according to a fifthembodiment.

FIG. 14 is a diagram showing the temperature response of the limitcurrent value in the sixth embodiment.

FIG. 15 is a diagram showing a linear power supply according to aseventh embodiment.

FIG. 16 is a diagram showing a linear power supply according to aneighth embodiment.

DESCRIPTION OF EMBODIMENTS

<Comparative Example>

First, prior to a description of linear power supplies (in particular,overcurrent protection circuits) according to novel embodiments, acomparative example to be compared with them will be described in brief.

FIG. 1 is a diagram showing a linear power supply of the comparativeexample. The linear power supply 1 of the comparative example includesan output transistor 10, a voltage divider 20, an amplifier 30, and anovercurrent protection circuit 100. The linear power supply 1 is a LDO(low drop-out) regulator that bucks (steps down) an input voltage VIN togenerate a desired output voltage VOUT. The input voltage VIN issupplied from a battery (not shown) or the like, and is not alwaysstable. The output voltage VOUT is supplied to a load 2 (such as asecondary power supply or a microprocessor) in a succeeding stage. Thelinear power supply 1 described above can be used, for example, as areference voltage source incorporated in a power supply IC.

The output transistor 10 is connected between an input terminal for theinput voltage VIN and an output terminal for the output voltage VOUT,and its conductance (reversely put, its on-state resistance value) iscontrolled by a gate signal G10 from the amplifier 30. In theillustrated example, used as the output transistor 10 is a PMOSFET(P-channel MOSFET). Accordingly, as the gate signal G10 decreases, theconductance of the output transistor 10 increases, and thus the outputvoltage VOUT rises; as the gate signal G10 increases, the conductance ofthe output transistor 10 decreases, and thus the output voltage VOUTlowers. Also usable as the output transistor 10 instead of a PMOSFET isan NMOSFET or a bipolar transistor.

The voltage divider 20 includes resistors 21 and 22 (with resistancevalues R1 and R2) connected in series between the output terminal forthe output voltage VOUT and a grounded terminal. The voltage divider 20yields, from the connection node between those resistors, a feedbackvoltage VFB (=VOUT×[R2/(R1+R2)]) proportional to the output voltageVOUT. In a case where the output voltage VOUT falls within the inputdynamic range of the amplifier 30, the voltage divider 20 may beomitted, in which case, as the feedback voltage VFB, the output voltageVOUT itself may be directly fed to the amplifier 30.

The amplifier 30 drives the output transistor 10 by generating the gatesignal G10 (corresponding to a drive signal for the output transistor10) such that the feedback voltage VFB, which is fed to thenon-inverting terminal (+) of the amplifier 30, remains equal to apredetermined reference voltage VREF, which is fed to the invertingterminal (−) of the amplifier 30. Specifically, as the difference ΔV(=VFB−VREF) between the feedback voltage VFB and the reference voltageVREF increases in the positive direction, the amplifier 30 raises thegate signal G10; as the difference ΔV increases in the negativedirection, the amplifier 30 lowers the gate signal G10.

The overcurrent protection circuit 100 controls the output of theamplifier 30 by generating an overcurrent protection signal Socp so asto limit the output current IOUT that passes through the outputtransistor 10 to or below a predetermined limit current value IOCP.

FIG. 2 is a diagram showing common overcurrent protection operation bythe overcurrent protection circuit 100. Along the horizontal axis istaken the output current IOUT, and along the vertical axis is taken theoutput voltage VOUT. As will be understood from the diagram, until theoutput current IOUT reaches the limit current value IOCP, the outputvoltage VOUT is kept at its target value. When the output current IOUTreaches the limit current value IOCP, current limiting takes effect, andthe output voltage VOUT falls from its target value.

As described above, the linear power supply 1 is provided with theovercurrent protection circuit 100 so that, even in the event of troublesuch as a short circuit at the output, it may not lead to thedestruction of the power supply IC in which the linear power supply 1 isintegrated. Needless to say, for reasons similar to that just mentioned,common power supply ICs (not only LDO regulators but also DC-DCconverters and the like) are regularly provided with an overcurrentprotection circuit.

<Problems With the Comparative Example>

FIGS. 3 and 4 are diagrams showing the temperature response (a firstpattern with a flat temperature response and a second pattern with anegative temperature response respectively) of the limit current valueIOCP in the comparative example. The broken lines in the diagramsrepresent the variation ΔIOCP of the limit current value IOCP.

For example, in a case where, as shown in FIG. 3, the limit currentvalue IOCP has a flat temperature response (i.e., where the limitcurrent value IOCP shows no temperature dependence), even when the chiptemperature Tj (junction temperature) of the power supply IC varies, thelimit current value IOCP does not vary (though slight variation has tobe tolerated because in practical terms it is not possible that thelimit current value IOCP has a perfectly flat temperature response).

In contrast, in a case where, as shown in FIG. 4, the limit currentvalue IOCP has a negative temperature response, as the chip temperatureTj rises, the limit current value IOCP decreases.

As described above, the limit current value IOCP of the overcurrentprotection circuit 100 is typically given a flat temperature response(see FIG. 3) or a negative temperature response (see FIG. 4) over anentire use temperature range (over a low to normal to high temperaturerange).

Here, when the overcurrent protection circuit 100 operates, it forciblyraises the on-resistance value of the output transistor 10 and therebylimits the output current IOUT to or below the limit current value IOCP.In this state, as the limit current value IOCP increases, the electricinverting input terminal (heat generation) in the output transistor 10increases.

FIG. 5 is a diagram showing a plot of common heat sinking. Along thehorizontal axis is taken the ambient temperature Ta, and along thevertical axis is taken the allowable package power dissipation Pd of apower supply IC. As shown there, in a temperature range equal to orhigher than normal temperature (25° C.), the allowable package powerdissipation Pd decreases as the ambient temperature Ta rises.

Accordingly, to prevent destruction of the power supply IC, it ispreferable that the limit current value IOCP be given a negativetemperature response such that, as the chip temperature Tj increases,the limit current value IOCP decreases, with the intention ofsuppressing the electric inverting input terminal (heat generation) inthe output transistor 10 in a high temperature range.

Inconveniently, giving the limit current value IOCP a negativetemperature response over an entire use temperature range (over a low tonormal to high temperature range) leads to a high limit current valueIOCP in a normal to low temperature range, and this complicates thedesigning of a set that incorporates the linear power supply 1. Forexample, in a configuration where a fuse is inserted in a stagepreceding the power supply IC, consideration needs to be given so thatthe limit current value IOCP in a low temperature range may not exceedthe fusing current of the fuse, and this leads to an excessive safetymargin for the limit current value IOCP in a high temperature range.

By contrast, giving the limit current value IOCP a flat temperatureresponse makes the designing of a set easy, but in practical use leadsto high heat generation in a high temperature range. This requires anadditional safety measure as by combining the overcurrent protectioncircuit 100 with an overheat protection circuit.

Presented below will be novel embodiments that can cope with theallowable package power dissipation of a power supply IC throughadequate overcurrent protection from a low to high temperature range.

First Embodiment

FIG. 6 is a diagram showing a linear power supply according to a firstembodiment. In the linear power supply 1 of this embodiment, theovercurrent protection circuit 100 is provided with a function ofadequately switching the temperature response of the limit current valueIOCP in accordance with the chip temperature Tj.

FIG. 7 is a diagram showing the temperature response of the limitcurrent value IOCP in the first embodiment. As shown there, theovercurrent protection circuit 100 gives the limit current value IOCP aflat temperature response when the chip temperature Tj is lower than athreshold value Tx and a negative temperature response when the chiptemperature Tj is higher than the threshold value Tx. The thresholdvalue Tx may be constant or variable.

Thus, with Tj<Tx (e.g., in a low to normal temperature range),irrespective of the chip temperature Tj, the variation of the limitcurrent value IOCP is kept small; in contrast, with Tj>Tx (e.g., in anormal to high temperature range), as the chip temperature Tj increases,the limit current value IOCP is lowered. In this way, it is possible toprevent destruction of the power supply IC in a high temperature range,and to prevent an excess output current IOUT and suppress variation ofthe limit current value IOCP in a low temperature range.

Second Embodiment

FIG. 8 is a diagram showing a linear power supply according to a secondembodiment. In the linear power supply 1 of this embodiment, theovercurrent protection circuit 100 includes a current detector 110 andan amplifier (or comparator) 120.

The current detector 110 detects the output current IOUT that passesthrough the output transistor 10, and generates a detection signal VSwith a flat temperature response. The current detector 110 can beprovided either upstream or downstream of the output transistor 10.

The amplifier (or comparator) 120 has a first input terminal (+) that isfed with the detection signal VS, a second input terminal (−) that isfed with a reference signal VREF_OPC1 with a flat temperature response,and a third input terminal (−) that is fed with a reference signalVREF_OPC2 with a negative temperature response. The amplifier (orcomparator) 120 generates an overcurrent protection signal Socp inaccordance with the difference between, or the result of comparisonbetween, whichever of the reference signals VREF_OPC1 and VREF_OPC2 islower and the detection signal VS.

This circuit configuration, i.e., one that employs a single amplifier(or comparator) 120 to determine the difference between, or comparebetween, the detection signal VS and the reference signal VREF_OPC1 orVREF_OPC2, is advantageous in terms of circuit area.

FIG. 9 is a diagram showing the temperature responses of the referencesignals VREF_OPC1 and VREF_OPC2 (top) and the temperature response ofthe limit current value IOCP (bottom).

As shown there, when the chip temperature Tj is lower than the thresholdvalue Tx, VREF_OCP1<VREF_OCP2. Accordingly the amplifier (or comparator)120 generates the overcurrent protection signal Socp in accordance withthe difference between, or the result of comparison between, thereference signal VREF_OPC1 and the detection signal VS. As a result, thelimit current value IOCP has a flat temperature response, andaccordingly, irrespective of the chip temperature Tj, the variation ofthe limit current value IOCP is kept small.

In contrast, when the chip temperature Tj is higher than the thresholdvalue Tx, VREF_OCP1>VREF_OCP2. Accordingly the amplifier (or comparator)120 generates the overcurrent protection signal Socp in accordance withthe difference between, or the result of comparison between, thereference signal VREF_OPC2 and the detection signal VS. As a result, thelimit current value IOCP has a negative temperature response, andaccordingly, as the chip temperature Tj rises, the limit current valueIOCP is lowered.

In this way, as the reference signal to be compared with the detectionsignal VS, two reference signals VREF_OPC1 and VREF_OPC2 with differenttemperature responses are used so that the limit current value IOCP canbe given, when the chip temperature Tj is lower than the threshold valueTx, a flat temperature response and, when the chip temperature Tj ishigher than the threshold value Tx, a negative temperature response.

In addition, through adjustment of the signal value of the referencesignal VREF_OPC1 and the gradient of the reference signal VREF_OPC2, thethreshold value Tx can be set at a desired value.

Third Embodiment

FIG. 10 is a diagram showing a linear power supply according to a thirdembodiment. In the linear power supply 1 of this embodiment, theovercurrent protection circuit 100 includes the current detector 110described previously along with amplifiers (or comparators) 121 and 122.

The amplifier (or comparator) 121 generates an overcurrent protectionsignal Socp1 in accordance with the difference between, or the result ofcomparison between, a reference signal VREF_OPC1 with a flat temperatureresponse, which is fed to the inverting input terminal (−) of theamplifier (or comparator) 121, and the detection signal VS, which is fedto the non-inverting input terminal (+) of the amplifier (or comparator)121.

The amplifier (or comparator) 122 generates an overcurrent protectionsignal Socp2 in accordance with the difference between, or the result ofcomparison between, a reference signal VREF_OPC2 with a negativetemperature response, which is fed to the inverting input terminal (−)of the amplifier (or comparator) 122, and the detection signal VS, whichis fed to the non-inverting input terminal (+) of the amplifier (orcomparator) 122.

This circuit configuration, i.e., one that employs two channels ofamplifiers (or comparators) 121 and 122 to determine the differencesbetween, or compare between, the detection signal VS and the referencesignals VREF_OPC1 and VREF_OPC2, provides more flexibility and freedomin the circuit designing of the overcurrent protection circuit 100.

For example, the amplifier (or comparator) 121, which operates in a lowtemperature range, is required to be power-saving rather thanfast-responding, and should preferably be designed to have lower currentconsumption than the amplifier (or comparator) 122, which operates in ahigh temperature range.

On the other hand, the amplifier (or comparator) 122, which operates ina high temperature range, is required to be fast-responding rather thanpower-saving, and should preferably be designed to have faster responsethan the amplifier (or comparator) 121, which operates in a lowtemperature range.

Fourth Embodiment

FIG. 11 is a diagram showing a linear power supply according to a fourthembodiment. In the linear power supply 1 of this embodiment, theovercurrent protection circuit 100 includes current detectors 111 and112 and an amplifier (comparator) 123.

The current detector 111 detects the output current IOUT that passesthrough the output transistor 10, and generates a detection signal VS1with a flat temperature response. The current detector 111 can beprovided either upstream or downstream of the output transistor 10.

The current detector 112 detects the output current IOUT that passesthrough the output transistor 10, and generates a detection signal VS2with a positive temperature response. The current detector 112 can beprovided either upstream or downstream of the output transistor 10.

The amplifier (or comparator) 123 has a first input terminal (+) that isfed with the detection signal VS1, a second input terminal (+) that isfed with the detection signal VS2, and a third input terminal (−) thatis fed with a reference signal VREF_OPC with a flat temperatureresponse. The amplifier (or comparator) 123 generates an overcurrentprotection signal Socp in accordance with the difference between, or theresult of comparison between, whichever of the detection signals VS1 andVS2 is higher and the reference signal VREF_OPC.

FIG. 12 is a diagram showing the temperature responses of the detectionsignals VS1 and VS2 (top), the temperature response of the referencesignal VREF_OPC (middle), and the temperature response of the limitcurrent value IOCP (bottom).

As shown there, when the chip temperature Tj is lower than the thresholdvalue Tx, VS1>VS2. Accordingly the amplifier (or comparator) 123generates the overcurrent protection signal Socp in accordance with thedifference between, or the result of comparison between, the referencesignal VREF_OPC and the detection signal VS1. As a result, the limitcurrent value IOCP has a flat temperature response, and accordingly,irrespective of the chip temperature Tj, the variation of the limitcurrent value IOCP is kept small.

In contrast, when the chip temperature Tj is higher than the thresholdvalue Tx, VS1<VS2. Accordingly the amplifier (or comparator) 123generates the overcurrent protection signal Socp in accordance with thedifference between, or the result of comparison between, the referencesignal VREF_OPC and the detection signal VS2. As a result, the limitcurrent value IOCP has a negative temperature response, and accordingly,as the chip temperature Tj rises, the limit current value IOCP islowered.

In this way, as the detection signal to be compared with the referencesignal VREF_OPC, two detection signals VS1 and VS2 with differenttemperature responses are used so that the limit current value IOCP canbe given, when the chip temperature Tj is lower than the threshold valueTx, a flat temperature response and, when the chip temperature Tj ishigher than the threshold value Tx, a negative temperature response.

In addition, through adjustment of the signal value of the detectionsignal VS1 and the gradient of the detection signal VS2, the thresholdvalue Tx can be set at a desired value.

Fifth Embodiment

FIG. 13 is a diagram showing a linear power supply according to a fifthembodiment. In the linear power supply 1 of this embodiment, theovercurrent protection circuit 100 includes the current detectors 111and 112 described previously along with amplifiers (or comparators) 124and 125.

The amplifier (or comparator) 124 generates an overcurrent protectionsignal Socp1 in accordance with the difference between, or the result ofcomparison between, a reference signal VREF_OPC with a flat temperatureresponse, which is fed to the inverting input terminal (−) of theamplifier (or comparator) 124, and the detection signal VS1, which isfed to the non-inverting input terminal (+) of the amplifier (orcomparator) 124.

The amplifier (or comparator) 125 generates an overcurrent protectionsignal Socp2 in accordance with the difference between, or the result ofcomparison between, the reference signal VREF_OPC, which is fed to theinverting input terminal (−) of the amplifier (or comparator) 125, andthe detection signal VS2, which is fed to the non-inverting inputterminal (+) of the amplifier (or comparator) 125.

This circuit configuration, i.e., one that employs two channels ofamplifiers (or comparators) 124 and 125 to determine the differencesbetween, or compare between, the detection signals VS1 and VS2 and thereference signal VREF_OPC, provides more flexibility and freedom in thecircuit designing of the overcurrent protection circuit 100. Itsworkings and benefits are similar to those achieved in the thirdembodiment described previously, and accordingly no overlappingdescription will be repeated.

Sixth Embodiment

FIG. 14 is a diagram showing the temperature response of the limitcurrent value IOCP in a sixth embodiment. In this embodiment, theovercurrent protection circuit 100 sets the limit current value IOCP,when the chip temperature Tj is lower than the threshold value Tx, at alimit current value IOCP1 that has a flat temperature response and, whenthe chip temperature Tj is higher than the threshold value Tx, at alimit current value IOCP2 that has a flat temperature response and thatis lower than the limit current value IOCP1.

Also with this circuit configuration, i.e., one that switches the limitcurrent value IOCP in accordance with the chip temperature Tj, as withthose of the first to fifth embodiments, it is possible to preventdestruction of the power supply IC in a high temperature range, and toprevent an excess output current IOUT and suppress variation of thelimit current value IOCP in a low temperature range. It should howeverbe noted that, around the switching point (Tj≈Tx) of the limit currentvalue IOCP, the overcurrent protection circuit 100 is prone to unstableoperation (such as oscillation of overcurrent protection operation).

Seventh Embodiment

FIG. 15 is a diagram showing a linear power supply according to aseventh embodiment. In the linear power supply 1 of this embodiment, theovercurrent protection circuit 100 is a more specific implementation ofthat in the second embodiment (FIG. 8) described previously. Theovercurrent protection circuit 100 here includes, in addition to thecurrent detector 110 and the amplifier (or comparator) 120, referencesignal generators 131 and 132 and a PMOSFET 140.

The current detector 110 includes a sense transistor Ms (e.g., aPMOSFET) and a sense resistor Rs. The source and the gate of the sensetransistor Ms are connected to the source and the gate, respectively, ofthe output transistor 10. The drain of the sense transistor Ms isconnected to the first terminal of the sense resistor Rs and, from theconnection node between these, the detection signal VS is yielded. Thesize ratio of the output transistor 10 to the sense transistor Ms is α:1 (where, e.g., α=10 000). Accordingly, through the sense transistor Mspasses a mirror current Im (=IOUT/α) proportional to the output currentIOUT. Passing the mirror current Im through the sense resistor Rs andthereby subjecting it to current-voltage conversion yields the detectionsignal VS (=Im×Rs).

The reference signal generator 131 generates the reference signalVREF_OPC1 with a flat temperature response by using a voltage (e.g., aband-gap voltage) that varies little with the chip temperature Tj.

The reference signal generator 132 includes a current source CS and adiode D that are connected in series between an application terminal forthe input voltage VIN and a grounded terminal, and output as thereference signal VREF_OPC2 the forward drop voltage Vf across the diodeD, which has a negative temperature response. Here, through adjustmentof the constant current generated by the current source CS, or throughinsertion of a buffer or a resistor ladder in a stage succeeding thereference signal generator 132, the gradient and offset of the referencesignal VREF_OPC2 can be set as desired.

The source of the PMOSFET 140 is connected to the application terminalfor the input voltage VIN. The drain of the PMOSFET 140 is connected toan application terminal for the gate signal G10 (i.e., the outputterminal of the amplifier 30). The gate of the PMOSFET 140 is connectedto an application terminal for the overcurrent protection signal Socp(i.e., the output terminal of the amplifier (or comparator) 120).

When the output current IOUT increases until VS>VREF_OCP1 (orVREF_OCP2), the overcurrent protection signal Socp falls and theon-state resistance of the output transistor 10 reduces. As a result,the gate signal G10 is pulled up and the on-state resistance of theoutput transistor 10 is forcibly raised; this invokes overcurrentprotection such that IOUT≤IOCP1 (or IOCP2).

Eighth Embodiment

FIG. 16 is a diagram showing a linear power supply according to aneighth embodiment. In the linear power supply 1 of this embodiment, theovercurrent protection circuit 100 is a more specific implementation ofthat in the third embodiment (FIG. 10) described previously. Theovercurrent protection circuit 100 here includes, in addition to thecurrent detector 110 and the amplifiers (or comparators) 121 and 122,reference signal generators 131 and 132 and PMOSFETs 141 and 142.

While in FIG. 16 a single current detector 110 is shared between theamplifiers (or comparators) 121 and 122, instead two current detectors110 may be provided, one for each of the amplifiers (or comparators) 121and 122.

The sources of the PMOSFETs 141 and 142 are both connected to theapplication terminal for the input voltage VIN. The drains of thePMOSFETs 141 and 142 are both connected to the application terminal forthe gate signal G10 (i.e., the output terminal of the amplifier 30). Thegates of the PMOSFETs 141 and 142 are connected respectively toapplication terminals for the overcurrent protection signals Socp1 andSocp2 (i.e., the output terminals of the amplifiers (or comparators) 121and 122).

If VREF_OCP1<VREF_OCP2, when the output current IOUT increases untilVS>VREF_OCP1, the overcurrent protection signal Socp1 falls and theon-state resistance of the PMOSFET 141 reduces. As a result, the gatesignal G10 is pulled up and the on-state resistance of the outputtransistor 10 is forcibly raised; this invokes overcurrent protectionsuch that IOUT≤IOCP1.

By contrast, if VREF_OCP2<VREF_OCP1, when the output current IOUTincreases until VS>VREF_OCP2, the overcurrent protection signal Socp2falls and the on-state resistance of the PMOSFET 142 reduces. As aresult, the gate signal G10 is pulled up and the on-state resistance ofthe output transistor 10 is forcibly raised; this invokes overcurrentprotection such that IOUT≤IOCP2.

Modified Examples

While all the embodiments described above deal with examples ofapplication to linear power supplies, this is not meant to limit to themthe application of overcurrent protection circuits according to thepresent invention, which needless to say find wide application in anycircuits that require an overcurrent protection function, such as powersupplies of any other types (e.g., switching power supplies) and inswitching circuits.

The various technical features disclosed herein may be implemented inany other manners than in the embodiments described above, and allow forany modifications made without departure from the spirit of theirtechnical ingenuity. That is, the embodiments described above should beconsidered to be in every aspect illustrative and not restrictive, andthe technical scope of the present invention should be understood to bedefined not by the description of the embodiments described above but bythe appended claims and to encompass any modifications made in a senseand scope equivalent to the claims.

INDUSTRIAL APPLICABILITY

The invention disclosed herein finds applications in vehicle-relatedappliances, nautical appliances, office appliances, portable appliances,smartphones, and the like.

REFERENCE SIGNS LIST

-   -   1 linear power supply    -   2 load    -   10 output transistor (PMOSFET)    -   20 voltage divider    -   21, 22 resistor    -   30 amplifier    -   100 overcurrent protection circuit    -   110, 111, 112 current detector    -   120, 121, 122, 123, 124, 125 amplifier (or comparator)    -   131, 132 reference signal generator    -   140, 141, 142 PMOSFET    -   CS current source    -   D diode    -   Ms sense transistor (PMOSFET)    -   Rs sense resistor

1. An overcurrent protection circuit for limiting a monitoring targetcurrent to or below a limit current value, the overcurrent protectioncircuit being configured to give the limit current value, whentemperature is lower than a threshold value, a flat temperature responseand, when temperature is higher than the threshold value, a negativetemperature response.
 2. The overcurrent protection circuit according toclaim 1, comprising: a current detector configured to detect themonitoring target current to generate a detection signal with a flattemperature response; and an amplifier or comparator configured to havea first input terminal fed with the detection signal, a second inputterminal fed with a first reference signal with a flat temperatureresponse, and a third input terminal fed with a second reference signalwith a negative temperature response, the amplifier or comparator beingconfigured to generate an overcurrent protection signal in accordancewith a difference between, or a result of comparison between, one of thefirst and second reference signals and the detection signal.
 3. Theovercurrent protection circuit according to claim 1, comprising: acurrent detector configured to detect the monitoring target current togenerate a detection signal with a flat temperature response; a firstamplifier or first comparator configured to generate a first overcurrentprotection signal in accordance with a difference between, or a resultof comparison between, a first reference signal with a flat temperatureresponse and the detection signal; and a second amplifier or secondcomparator configured to generate a second overcurrent protection signalin accordance with a difference between, or a result of comparisonbetween, a second reference signal with a negative temperature responseand the detection signal.
 4. The overcurrent protection circuitaccording to claim 3, wherein the first amplifier or first comparatorhas lower current consumption than the second amplifier or secondcomparator, and the second amplifier or second comparator has fasterresponse than the first amplifier or first comparator.
 5. Theovercurrent protection circuit according to claim 2, further comprising:a first reference signal generator configured to generate the firstreference signal by using a band-gap voltage.
 6. The overcurrentprotection circuit according to claim 2, further comprising: a secondreference signal generator configured to generate the second referencesignal by using a forward drop voltage across a diode.
 7. Theovercurrent protection circuit according to claim 2, wherein the currentdetector is configured to generate the detection signal by passing amirror current proportional to the monitoring target current through asense resistor and thereby subjecting the mirror current tocurrent-voltage conversion.
 8. The overcurrent protection circuitaccording to claim 1, comprising: a first current detector configured todetect the monitoring target current to generate a first detectionsignal with a flat temperature response; a second current detectorconfigured to detect the monitoring target current to generate a seconddetection signal with a positive temperature response; and an amplifieror comparator configured to have a first input terminal fed with thefirst detection signal, a second input terminal fed with the seconddetection signal, and a third input terminal fed with a reference signalwith a flat temperature response, the amplifier or comparator beingconfigured to generate an overcurrent protection signal in accordancewith a difference between, or a result of comparison between, one of thefirst and second detection signals and the reference signal.
 9. Theovercurrent protection circuit according to claim 1, comprising: a firstcurrent detector configured to detect the monitoring target current togenerate a first detection signal with a flat temperature response; asecond current detector configured to detect the monitoring targetcurrent to generate a second detection signal with a positivetemperature response; a first amplifier or first comparator configuredto generate a first overcurrent protection signal in accordance with adifference between, or a result of comparison between, a referencesignal with a flat temperature response and the first detection signal;and a second amplifier or second comparator configured to generate asecond overcurrent protection signal in accordance with a differencebetween, or a result of comparison between, the reference signal and thesecond detection signal.
 10. An overcurrent protection circuit forlimiting a monitoring target current to or below a limit current value,the overcurrent protection circuit being configured to set the limitcurrent value, when temperature is lower than a threshold value, at afirst limit current value that has a flat temperature response and, whentemperature is higher than a threshold value, at a second limit currentvalue that has a flat temperature response and that is lower than thefirst limit current value.